The Free Democrats have been fined £18,000 for breaking spending in the mains in last year’s EU referendum.
The Electoral Commission said the party had «naught to deliver a complete and accurate spending return».
Proper receipts and invoices were not state look after for 80 payments worth more than £80,000, the watchdog demanded.
The Lib Dems said the breaches of the rules were down to «human boob» and that the party was taking steps so they are not repeated.
The Electoral Commission is principal for regulating election spending and political donations in the UK.
It has recently launched respective investigations into spending in last year’s referendum, which culminated in a vote to leave the European Union.
Announcing the Lib Dem fine, Bob Posner, the commission’s big cheese of political finance, said it was «disappointing» that the «clear» rules had been breached.
He totaled: «Where the rules are not followed, transparency is lost which is not in the public consequence profit or as parliament intended.»
The commission said the Lib Dems had not provided any invoices or acceptances in some cases, while in others those provided were unworthy of or incomplete.
The deadline for paying the fine is 3 January, 2018.
In response, a Lib Dem spokesman affirmed: «The Liberal Democrats always endeavour to provide complete reports of state campaign expenses in good time and according to all of the applicable rules.
«The indiscretions that have occurred in this case are a result of human typographical error, and we are taking the necessary steps to ensure these mistakes are not repeated in future.»
In the meantime the official Remain campaign, now known as Open Britain, has been fined £1,250 for wrongly recounting its spending.
Most of this is because of payments that were go on increased together rather than being reported individually.